Snap scaling doesn't behave like expected
Don’t worry, it works.

1) Enable snapping to vertex
2) When using a scale tool grab a vertex of your circle to scale it. Grabbing the scale tool ring won’t work.
3) The vertex you’re holding is now snapping to other vertices.

Enable snapping to vertex


Grab a vertex, not the scale tool ring


Moving the vertex now scales the object and snaps to other vertices


Hope this helps.
If you need any clarification, let me know.